UTRAN (Universal Terrestrial Radio Access Network) is a term that identifies the radio access network of a UMTS (Universal Mobile Telecommunications System), wherein the UTRAN consists of Radio Network Controllers (RNCs) and NodeBs i.e. radio base stations. The NodeBs communicate wirelessly with mobile user equipments and the RNCs control the NodeBs. The RNCs are further connected to the Core Network (CN). Evolved UTRAN (E-UTRAN) is an evolution of the UTRAN towards a high-data rate, low-latency and packet-optimised radio access network. Further the E-UTRAN consists of NodeBs, and the NodeBs are interconnected and further connected to the Evolved Packet Core network (EPC). E-UTRAN is also being referred to as Long Term Evolution (LTE) and is standardized within the 3rd Generation Partnership Project (3GPP).
The third generation cellular systems such as WCDMA (Wideband Code Division Multiple Access) may be equipped with a number of different positioning methods, thereby enabling location services to the cellular subscribers. These methods are generally applicable also in wireless communications systems using other radio access technologies, such as the LTE. The methods that are available include                Cell identity (Cell-ID) positioning.        Enhanced cell identity (Ecell-ID) positioning        Assisted GPS (A-GPS) positioning        Downlink time difference of arrival—with idle periods in the downlink (OTDOA-IPDL) positioning        Uplink time difference of arrival (UTDOA) positioning        
Cell-ID positioning determines the cell to which the user equipment (UE) is connected. The position of the user is hence determined with cell granularity. Typically the radio network controller of the radio network (RAN) determines a 3-15 corner polygon that determines the geographical extension of the cell. The corners of this polygon are given as latitude, longitude pairs in the WGS84 geographical reference system. The cell-ID method is the backbone of all cellular positioning system since it is always available when the UE can be connected to the system.
Ecell-ID positioning augments the Cell-ID positioning with auxiliary information that narrows down the area that is determined by the cell polygon. A useful method in the WCDMA system is the round trip time (RTT) measurement. This measurement determines the travel time back and forth from the radio base station (RBS) to the UE and back. Using the speed of light, the distance from the known position of the RBS to the UE can be calculated, which determines a circular strip around the RBS where the UE is located. The thickness of the strip is determined by the measurement uncertainty. The Ecell-ID method is obtained by noticing that the UE is located both in the cell and in the circular strip. Hence, the UE is located in the intersection of these two geographical regions.
A-GPS positioning is an enhancement of the US military global positioning system (GPS). GPS reference receivers attached to e.g. a cellular communication system, collect assistance data that, when transmitted to GPS receivers in terminals connected to the cellular communication system, enhances the performance of the GPS terminal receivers. Typically, A-GPS accuracy can become as good as 10 meters also without differential operation. The accuracy becomes worse in dense urban areas and indoors, where the sensitivity is often not high enough for detection of the very weak signals from the GPS satellites. Advantages of A-GPS includes a high accuracy, the method easily meets the North-American emergency positioning E-911 requirements of 50 meters for 67% of all positionings and 150 meters for 95% of all positionings. A drawback is the limited indoor coverage, which is a result of the low ranging signal strengths that are obtained at ground level.
OTDOA-IPDL positioning is similar to A-GPS in that it relies on time difference of arrival measurements. However, the OTDOA-IPDL method uses UE measurements of Pilot radio (CPICH in WCDMA) signals transmitted from several RBSs. The measurement results are signalled to the RNC, where a hyperbolic trilateration method is used for calculation of the position of the UE. In order to enhance the hearability of the RBSs in the UE, there is a possibility to use idle periods in the downlink (IPDL), to attenuate the transmissions from the RBS to which the UE is connected. This reduces the interference and hence enhances the hearability of other RBSs. A tentative advantage with OTDOA-IPDL is that it theoretically provides a better indoor coverage than does A-GPS.
UTDOA positioning is another positioning method. It is similar to A-GPS in that it relies on time difference of arrival measurements. However, the UTDOA method uses RBS (or separate location measurement unit (LMU)) measurements of signals transmitted from the positioned UE. The transmitted signal is detected in a number of RBSs or LMUs, after which the measured results are signalled to a positioning node where the position of the UE is determined by a trilateration method. In order to be able to detect the time of arrival from measurements of opportunity from the UE, a reference signal first needs to be created in a master-LMU or master RBS. This is done by decoding of the signal, followed by reconstruction of the chip stream that then forms the reference signal. An advantage of UTDOA positioning is that it provides a better indoor coverage than does A-GPS. Outdoor accuracy is normally inferior to A-GPS though.
An issue with terrestrial time difference of arrival methods, i.e. OTDOA-IPDL and UTDOA, is the receiver sensitivity when positioning is considered. Theoretically, the methods can provide a 3-D position from 4 times of arrival measurements (equivalent to three time difference of arrival (pseudo)measurements). However, radio propagation conditions are far less beneficial than for A-GPS, since OTDOA-IPDL and UTDOA ranging signals propagate along the surface of the earth, whereas A-GPS signals propagate from above. The terrestrial positioning methods therefore suffer more from non-line-of-sight (LOS) propagation and multipath propagation. This results in outlier measurements, whose suppression requires the availability of excess detections i.e. detections from significantly more than the minimum number of RBSs. In practice, to achieve a useful positioning accuracy, at least 6-8 RBSs need to be detected in the UE in case OTDOA-IPDL positioning is used. For UTDOA positioning at least 6-8 RBSs need to detect the UE transmissions in order to obtain useful position estimates in practical environments.
The consequence of the above is that more remote RBSs need to be detected (OTDOA-IPDL) or detect (UTDOA). This means that lower signal strengths need to be detected with high probability. Calculations typically show that signals need to be detected down to about −40 dB C/I. Further, the pre-detection step needs to enhance the signal to about 11-13 dB C/I in order to achieve a sufficiently low false alarm rate. In essence, the processing gain for positioning purposes in any CDMA system needs to be 50-55 dB for terrestrial positioning to be useful. This is significantly more than what is needed for other services, which means that positioning sensitivity requirements need to be assessed at the definition phase of the air-interface.
For LTE, a possible positioning technique is Downlink OTDOA, using UE measurements on measurement signals, e.g. reference signals and/or synchronization signals, on multiple cells during designated low interference subframes, an example being MBSFN (Multicast Broadcast Multimedia Single Frequency Network) subframes.
However, the main problem of the third generation cellular systems positioning methods remains and can be summarized as follows:                A-GPS positioning is a high precision technology with one main drawback—indoor positioning availability.        OTDOA-IPDL and UTDOA positioning have the technical potential to provide better indoor coverage than A-GPS and to deliver good precision. However, the presently available detection sensitivities are not sufficient to provide a good enough accuracy. Rather, accuracy figures in between A-GPS and Cell-ID can be expected.        
Thus, a problem in current positioning solutions for wireless communications systems such as LTE is that it is difficult to achieve sufficient positioning accuracy with reasonable receiver detection sensitivities.